This invention relates to a hydraulically-operated friction brake of the kind wherein the hydraulic liquid also provides the medium for removing the heat generated during operation of the brake. The construction and operation of one such friction brake is disclosed in our German Offenlegungschrift 2446669 and in the U.S. Patent Application Ser. No. 510,126 directly corresponding thereto. Said U.S. Application is now U.S. Pat. No. 3,924,712, which claims priority from the same eight British Patent Applications as does said German Offenlegungschrift.
In general terms, the friction brake of the aforesaid Offenlegungschrift comprises (i) a casing, (ii) a rotary member mounted in the casing, (iii) a pressure-operable retarding device arranged and adapted to act upon the rotary member to generate retarding torque. (iv) a pump for circulating hydraulic liquid, (v) two separate fluid circuits provided between the pump and the retarding device, one being an actuating circuit arranged and adapted to operate the retarding device, and (vi) valve means comprising (a) a valve member, (b) a first valve elememt secured to said member, (c) a second valve element slidable on said member and urged away from the first valve element by a spring and constituting pressure sensitive means arranged and adapted to control distribution of flow of hydraulic liquid between the two circuits, and (d) means for equilibrating hydraulic pressure across the valve means.
More specifically, the brake just recited comprises a casing, a rotary member housed therein and connectable to a vehicle propellor shaft and associated therewith a pressure-operated retarding device (iii) comprising (a) annular friction plates mounted on the rotary member, (b) annular stator plates mounted on the casing and interleaved with the friction plates, the stator and friction plates (collectively referred to hereafter as the `pack`) being relatively axially slidable under the action of (c) an hydraulically-operable actuator in the form of a piston-and-cylinder device or bellows piston to bring the stator and friction plates into contact and generate therebetween a retarding torque on the rotatable member. A pump circulates hydraulic liquid through at least one hydraulic circuit which includes the pack and a cooling means and which has a branch through which hydraulic liquid is supplied to operate the actuator piston when it is desired to operate the retarding device. In the arrangement disclosed, the casing is filled with hydraulic liquid, leaving only such space as is necessary to accommodate the increase in the volume of the hydraulic liquid which occurs when the liquid absorbs heat during periods of actuation of the retarding device, and the hydraulic liquid is circulated through the pack even during periods of non-actuation of the retarding device as long as the vehicle to which the brake if fitted is in motion. This continuous circulation of at least a major proportion of the hydraulic liquid through the pack causes a significant amount of `drag` with absorption of power which could be used more efficiently elsewhere.